This invention relates to a superconductor having a high critical temperature and a high critical electrical current and to a method of producing same.
Recent development of a melt processing technique has enabled the production of excellent superconductors. One such melt processing technique is a melt-powder-melt-growth (MPMG) process by which a superconductor containing a Y.sub.2 BaCuO.sub.5 (Y211) phase dispersed in a YBa.sub.2 Cu.sub.3 O.sub.y (Y123) phase and having a high critical electrical current can be obtained ("Melt Processed High Temperature Superconductor", M. Murakami, World Scientific, 1993). Such a superconductor can create a large electromagnetic force upon interaction with a magnetic field and can be utilizable for bearings, flywheels, transporting devices, etc.
Another known melt processing technique is a process in which a large crystal, such as of Y123, having controlled crystal orientation is grown using a seed crystal such as of La123, Nd123 or Sm123. A large crystal with controlled crystal orientation is desirable for applications to magnetic shields and permanent magnets.
Known RE123 (RE: rare earth element) superconductors have a serious problem that part of the Ba site is substituted by RE ion which has a large ionic radius so that the critical temperature is lowered (H. Ywe et al, Physica C, vol. 153-155, p. 930-931 (1988)).
U.S. Pat. No. 5,525,584 discloses a process for the production of a metal oxide superconductor having the formula (R.sup.1.sub.1-x,Ba.sub.x)Ba.sub.2 Cu.sub.3 O.sub.d (where R.sup.1 is an element selected from La, Nd, Sm, Eu and Gd and d is a number between 6.2 and 7.2), wherein a melt having a temperature of 1,000-1,300.degree. C. and containing R.sup.1, Ba, Cu and O in such amounts that the content of R.sup.1 is 0.3-0.6 mole, the content of Ba is 0.6-0.8 mole and the content of O is 2.1-2.7 moles each per mole of Cu is cooled at a cooling rate of 5.degree. C./hour or less under a partial pressure of oxygen of between 0.00001 and 0.05 atm.
While this method can give a metal oxide superconductor having a critical temperature of 90K or more, there is a problem because it is necessary to strictly control the oxygen partial pressure and because the crystal growth velocity is so slow that the production cost is highly increased.